Mill behavior was examined with gum rubbers and carbon black-filled compounds of different types of cis-1,4-polybutadiene. The rubbers were Ti-, Nd-, and Copolymerized polymers. The Co-polymerized rubber contained crystalline particles of 1,2-polybutadiene. The carbon black was 50 phr N330. Photographs of the samples on the mill were taken after milling for 2 min. The mill behaviors were classified according to the four regions of Tokita and White. Behavior at loading, stickiness, and mill shrinkage were evaluated qualitatively to interpret mill behavior with aid of the deformational data of previous publications. The Ti-rubber, containing relatively short branches and exhibiting a strain-softening, was in Region III. The Nd-rubbers, containing relatively long branches and exhibiting a strain-hardening, tended to go to Region IV, because the degree of branching was low. The Co-rubbers containing crystalline particles were in Region II, even though they gave strain-softening since they had a high modulus because of the presence of the particles. An overriding effect on the mill behavior appeared to come from how high the modulus of the gum rubber was at the milling condition. Both the gum rubber and the corresponding compound were in the same region of the mill processability. The strain-induced crystallization occurred only when the compounds were under tension on the mill, i.e., Region II.